The polymerization of olefins in a fluidized gas phase reactor is typically carried out using a seedbed. In the case of polyethylene, one of two types of seedbed is used: a linear low-density polyethylene (LLDPE) seedbed or a high-density polyethylene (HDPE) seedbed. In the case of polymerization of other olefins, other suitable types of seed beds may be used. In the polyethylene process, the low density seedbeds may absorb more heavy hydrocarbons than the high density seedbeds, resulting in an increase in the formation of agglomerates or sheeting on the reactor wall. As a result, the higher density seedbeds have typically been used when starting-up a reactor for polymerizing olefins. However, the high density seedbeds may generate high amount of waste and subproducts, resulting in lost production time to remove these substandard products.
The formation of agglomerates and sheeting on the reactor walls may be explained by various factors. Agglomerates and sheeting may form when the polymerization temperature is too close to the polymer sintering temperature (temperature at which agglomerates start forming) or when the polymer particles become excessively sticky. Highly active fine particles may concentrate in the upper elevations of the polymerization zone, towards the top of the fluidized bed and in the powder disengagement zone above the bed, thus leading to local hot spots and potential agglomeration and sheeting. The accumulation of polymer at the reactor walls can be as thin as a few micrometers to several centimeters.
Agglomeration and sheeting are responsible for costly production losses, unreliable operation, which may be unsafe, and limit overall plant performance. The need to clean the reactor and any other maintenance issues related to sheeting and agglomeration formation result in unplanned reactor downtime, leading to reduced production and revenue. Therefore, a need exists to provide additional methods of controlling agglomeration and sheeting formation to decrease the amount of off-specification products as well as improving the economics of the gas phase reactor process.